National Aeronautics and Space Administration Triton Dione Tethys Iapetus Mimas Rhea Enceladus Titan Earth’s Moon Earth Europa Titania Miranda Oberon Callisto Io Charon Ganymede Moons of the Solar System www.nasa.gov Moons — also called satellites — come in many shapes, sizes, In the realm beyond Saturn, Uranus has 27 known moons. The SIGNIFICANT DATES and types. They are generally solid bodies, and few have atmo- inner moons appear to be about half water ice and half rock. 1610 — Galileo Galilei and Simon Marius independently discover spheres. Most of the planetary moons probably formed from the Miranda is the most unusual; its chopped-up appearance shows four moons orbiting Jupiter. Galileo is credited and the moons discs of gas and dust circulating around planets in the early solar the scars of impacts of large rocky bodies. Neptune’s moon are called “Galilean.” system. As of September 2009, astronomers have found at least Triton is as big as the dwarf planet Pluto, and orbits backwards 1877 — Asaph Hall discovers Mars’ moons Phobos and Deimos. 145 moons orbiting planets in our solar system. This number compared with Neptune’s direction of rotation. Neptune has 1969 — Astronaut Neil Armstrong is the first of 12 humans to does not include the six moons of the dwarf planets, nor does 13 known moons. walk on the surface of Earth’s Moon. this tally include the tiny satellites that orbit some asteroids and Pluto’s large moon, Charon, is about half the size of Pluto. Like 1980 — Voyager 1 instruments detect signs of surface features other celestial objects. Earth’s Moon, Charon may have formed from debris resulting beneath the hazy atmosphere of Saturn’s largest moon, Titan. Of the terrestrial (rocky) planets of the inner solar system, neither from an early collision of an impactor with Pluto. In 2005, sci- 2000–present — Using improved ground-based telescopes, Mercury nor Venus has any moons at all, Earth has one, and entists using the Hubble Space Telescope to study Pluto found the Hubble Space Telescope, and spacecraft observations, sci- Mars has its two small moons. In the outer solar system, the gas two additional, but very small, moons. The little moons (Nix and entists have found dozens of new moons in our solar system. giants (Jupiter, Saturn) and the ice giants (Uranus and Neptune) Hydra) are about two to three times as far from Pluto as Charon Newly discovered moons (as well as other solar system objects) have numerous moons. As these planets grew in the early solar and roughly 5,000 times fainter than Pluto. Eris, another dwarf are given temporary designations until they are confirmed by system, they were able to capture objects with their large gravi- planet even more distant than Pluto, has a small moon of its subsequent observations and receive permanent designations tational fields. own, named Dysnomia. Haumea, another dwarf planet, has two from the International Astronomical Union. satellites, Hi’iaka and Namaka. Earth’s Moon probably formed when a large body about the size ABOUT THE IMAGES of Mars collided with Earth, ejecting a lot of material from our FAST FACTS — PlAneTS & SigniFiCAnT MoonS 1 2 1 Selected solar planet into orbit. Debris from the early Earth and the impacting system moons, dis- Mean Radius Mean Radius body accumulated to form the Moon approximately 4.5 billion playing a variety of Planet Moon (km) (mi) 3 years ago (the age of the oldest collected lunar rocks). Twelve surface features, are American astronauts landed on the Moon during NASA’s Apollo Earth Moon 1,737.4 1,079.6 shown at correct rela- program in 1969 to 1972, studying the Moon and bringing back 4 6 tive sizes to each other rock samples. Mars Phobos 11.1 6.9 5 and to Earth. Mars Deimos 6.2 3.9 Usually the term “moon” brings to mind a spherical object, like 2 Miranda, a moon of Uranus, has many rugged features: Earth’s Moon. The two moons of Mars, Phobos and Deimos, are Jupiter Io 1,821.6 1,131.9 canyons, grooved structures, ridges, and broken terrain. somewhat different. While both have nearly circular orbits and Jupiter Europa 1,560.8 969.8 3 This false-color image of Neptune’s moon Triton shows what travel close to the plane of the planet’s equator, they are lumpy Jupiter Callisto 2,410 1,498 appear to be volcanic deposits. and dark. Phobos is slowly drawing closer to Mars, and could Jupiter Ganymede 2,631 1,635 crash into Mars in 40 or 50 million years, or the planet’s gravity 4 This Voyager 1 close-up of Saturn’s moon Rhea shows the Saturn Mimas 198.6 123.4 might break Phobos apart, creating a thin ring around Mars. moon’s ancient, cratered surface. Saturn Enceladus 249.4 154.9 5 A portion of a Cassini radar image of Saturn’s largest moon, Jupiter has 49 known moons (plus 13 awaiting official confirma- Saturn Tethys 529.9 329.3 Titan, showing the complexity of the surface. tion), including the largest moon in the solar system, Ganymede. Saturn Dione 560 348 Many of Jupiter’s outer moons have highly elliptical orbits and Saturn Rhea 764 475 6 Cassini imaged the small irregular moon Phoebe when the orbit “backwards” (opposite to the spin of the planet). Saturn, Saturn Titan 2,575 1,600 spacecraft was inbound for Saturn orbit insertion in June 2004. Uranus, and Neptune also have some “irregular” moons, which Saturn Iapetus 718 446 orbit far from their respective planets. FOR MORE INFORMATION Uranus Miranda 235.8 146.5 solarsystem.nasa.gov/planets/profile.cfm?Object=Moons Saturn has 53 known moons (plus 9 awaiting official confirma- Uranus Ariel 578.9 359.7 tion). The chunks of ice and rock in Saturn’s rings (and the par- Uranus Umbriel 584.7 363.3 ticles in the rings of the other outer planets) are not considered Uranus Titania 788.9 490.2 moons, yet embedded in Saturn’s rings are distinct moons or Uranus Oberon 761.4 473.1 “moonlets.” “Shepherd” moons help keep the rings in line. Sat- Neptune Triton 1,353.4 841 urn’s moon Titan, the second largest in the solar system, is the Neptune Nereid 170 106 only moon with a thick atmosphere. LG-2009-09-572-HQ — JPL 400-1344K 09/09.